Environmental protection requires a considerable reduction of the emission of nitrogen oxides in waste gases. As an example, the present limit set by the European Community directives require a reduction of the content of nitrogen oxides to a maximum of 200 mg/m.sup.3 (calculated as NO.sub.2) in flue gases from large scale power stations. These low concentrations cannot be obtained only by special measures during the combustion process (so-called primary measures) but require the application of special processes for the removal of nitrogen oxides from waste gases, i.e. denoxing processes.
The process for the removal of nitrogen oxides consists in the reduction of the compound to nitrogen. For flue gases, several industrial processes are already known (Chem. Ing. Techn. 57, 1985, page 717 to 727).
The presence of nitrates in waste waters is also a growing problem. The intensive use of natural manure and fertilizers in agriculture leads to an increasing content of nitrates in subsoil water. In many places in Europe, drinking water producers have difficulties in maintaining the limit below 50 mg/m.sup.3 of nitrates in their final product.
The usual way of removing nitrates from waste waters is by biological processes which are slow and expensive. EP-A-0 243 889 discloses a process for the electrolytic denoxing of flue gases. The nitrogen oxides are absorbed in a iron-ethylene diamine-tetraacetic acid (Fe-EDTA) complex. The reduction occurs then according to the following reaction equation: EQU 2 Fe(NO)EDTA+2H.sup.+ +2e.fwdarw.2Fe(EDTA)+N.sub.2 +H.sub.2 O(1)
However, in practice it has been found, that the electrolytic reduction of this complex leads to the formation of ammonia and, as an intermediate product, of hydroxylamine according to the following reaction equations: EQU Fe(NO)(EDTA)+3 H.sup.+ +3 e.fwdarw.Fe(EDTA)+NH.sub.2 OH (2) EQU Fe(NO)EDTA+5 H.sup.+ +5 e.fwdarw.Fe(EDTA)+NH.sub.3 +H.sub.2 O(3)
For the development of an attractive denoxing process, the formation of ammonia in the catholyte is not desired since the removal of the ammonia from this liquid presents new problems. Therefore it would be highly desirable that the nitrogen oxides could be converted into gaseous nitrogen.
It is already known that NH.sub.3 can be oxidized to nitrogen by chemical oxidation with hypobromides and hypochlorides according to the following reactions: EQU 3BrO.sup.- +2NH.sub.3 .fwdarw.N.sub.2 +3Br.sup.- +3H.sub.2 O(4)
If a solution containing ammonia and bromide ions is submitted to electrolysis, primarily bromine will be formed at the anode: EQU 2Br.sup.- .fwdarw.Br.sub.2 +2 e (5)
It is further known from French Patent 1 493 735 to produce bromine by electrolysis in a cell which is derided by a microporous fabric into two reaction chambers. Bromine production in an electrolytic cell is also mentioned in the first quoted document EP-A 0 243 889.
As bromine is unstable in alkaline solutions, the following disproportionation reaction occurs: EQU Br.sub.2 +2OH.sup.- .fwdarw.Br.sup.- +BrO.sup.- +H.sub.2 O (6)
Subsequently, the formed hypobromide oxidizes ammonia to nitrogen according to reaction (4).
In acidic solutions, bromine is stable and oxydizes ammonia according to the following reaction: EQU 3Br.sub.2 +8NH.sub.3 .fwdarw.N.sub.2 +6NH.sub.4.sup.+ +6Br.sup.- (7)
It thus follows that the presence of bromine in alkaline or acid solutions in the anodic compartment leads to the chemical oxidation of ammonia and the formation of gaseous nitrogen. However, it must be noted that during the electrolysis reduction, ammonia is formed in the cathodic compartment.